Plants naturally produce an assortment of fatty acids and synthesize an even wider assortment of lipids, including mono-, di- and triacylglycerols, phospholipids, glycolipids, and others, from the fatty acids they produce. The specific assortment of lipids made by any particular plant is determined by both the genotype of the plant and the plant's response to environmental factors such as heat, cold, drought, etc. However, regardless of the environmental conditions, a plant can never produce a fatty acid or lipid composition for which it does not have the necessary biochemical machinery, and such biochemical pathways are ultimately determined by genotype. Traditional methods of genetic modification involve genetic recombination processes which are directed by the plant breeder at the whole plant level. These methods, while well characterized and straightforward to conduct, typically produce incremental improvements in oil content and composition by optimizing the native biochemistry, rather than by creating new biochemical pathways.
At the same time, because of their influence on food quality and significance in biological processes, there is continuing interest in the alteration of fatty acid desaturation mechanisms in plants. The properties of fats and oils are determined by their fatty acid composition, which in turn affects nutritional quality and oxidative stability. Likewise, the specific structures and compositions of other plant lipids which the plant synthesizes from fatty acids are dependent upon the makeup of the fatty acid pool which is available as precursors to the biosynthesis of those lipids.
Recently there has been interest in reducing the content of saturated fatty acids in foods. Medical and nutritional research have led many food and food component producers to want certain compositions in their fat and oil based foods and food components. Those desired compositions are frequently high in mono- and polyunsaturated fatty acids and corresponding triacylglycerol stores, or are low in saturated fatty acids and saturated fatty acid-based triacylglycerols. Industrial users of plant-derived fats and oils also have preferences for the specifications of feedstocks used in their industrial processes, and such specifications often call for large percentages of a single fatty acid moiety. Often the preferred fatty acid moiety is an unsaturated fatty acid moiety such as palmitoleate, oleate, linoleate, or linolenate. Unfortunately, nature does not cooperate by providing oilseed plants which produce the preferred compositions. Efforts have therefore been initiated to develop oilseed varieties and hybrids which yield vegetable oils with higher monounsaturated fatty acid contents. However, in view of the incremental nature of whole-plant genetic methods, the need and desire continue to exist for compositions and methods which can affect and create biochemical pathways at the single-gene level through genetic engineering.
Even when traditional plant breeding methods are successful in altering fatty acid composition in the lipids of a plant variety, the native biochemical pathways of the plant still exhibit all of their art-recognized characteristics and limitations. Thus, for example, oilseed crops which have been improved by plant breeding exhibit the usual responses to environmental variations. These responses include a tendency to produce higher percentages of saturated fats under warmer growing conditions and higher percentages of unsaturated fats under cooler growing conditions, making the reliable production of oilseeds having a particular fatty acid composition as difficult as predicting the weather. Thus, it would also be highly desirable to have means for compensating for these environmental influences. Little effort has been invested to date toward that objective.
Finally, there is a continuing desire to improve and extend the environmental range of crop plants. Some oilseed species originated in temperate, subtropical and tropical regions and are poorly adapted to cooler production areas. Even oilseeds which are suitable for cooler climates can benefit from further adaptation, since moving up the planting time in the spring and extending the growing season in the summer and fall can sometimes be exploited for higher crop yields. Plant breeders have focused a great deal of attention on one aspect of the necessary climatic adaptation, maturation rate, but another important aspect of climatic adaptation is chilling tolerance (as distinguished from freezing tolerance.)